Frequency hopping is a radio communication technique in spread-spectrum modulation wherein information is transmitted using a sequence of carrier frequencies that change at set times to produce a narrow band signal that bounces or hops around in center frequency over the available spectrum.
In a centrally controlled multicellular mobile radio communication system based on slow frequency hopping, each cell has a base station that provides the necessary timing and control information received and used by all the remote stations that belong to the cell.
All stations belonging to a cell, the base station and all remote stations that belong to it, must hop in synchronism in order to communicate with each other at the same frequency. Different cells will typically operate on different frequency hopping patterns, The control information required for synchronized frequency hopping is broadcast by the base station. A key problem in the operation of a frequency-hopping based system is that of maintaining hop synchronization between all stations that belong to the same cell. Synchronization must be ensured even under conditions of loss of transmission of control information. The problem of maintaining synchronism can be further divided into the subproblems of: a) Obtaining initial synchronism, b) Staying in synchronism and c) Reacquisition of synchronism after temporary loss of synchronism. The base station may modify at any time the frequency hopping pattern (for instance to overcome interferences) and the remote stations must be able to follow this pattern change in an efficient and reliable manner.
The following references are typical of the background art in the field of frequency hopping systems and synchronization techniques therefor.
In U.S. Pat. No. 5,130,987 issued Jul. 14, 1992 to Flammer entitled "Method For Synchronizing A Wide Area Network Without Global Synchronizing", a frequency-hopping packet communication system without a master clock or master control unit is described which is based on use of a receiver's frequency hopping timing and identification to control communication. A frequency-hopping band plan, involving the number of channels and the pseudo-random pattern of frequency change and nominal timing of changes, is universally known to each node in the network. A transmitter acquires synchronization with a target node by use of information previously received from or about a target indicating timing of present idle frequency hop of the target receiver. Each receiving node establishes in each station or node a table of receiver frequency hopping sequence offsets (hop timing offsets) of each other node within its communication range, and each node announces its communication range, and each node announces its presence on each frequency in a packet with a hop timing offset indicator. The hop timing offset indicator is a key used to read a table to allow nodes to set themselves in synchronization with one another. A location indicator built into the address of each packet is used to randomize an ordered frequency-hopping table at each node.
Frequency-hopping is implemented by the division of communication slots and the accumulation of slots into epochs, wherein each epoch equals the total number of available slots (number of channels times the number of time frames per channel). The transmitting node tracks the pre-established frequency-hopping pattern for its target receiver based on previously-acquired information.
U.S. Pat. No. 5,121,408 issued Jun. 9, 1992 to Cai et al. entitled "Synchronization For Entry To A Network In A Frequency Hopping Communication System" discloses techniques for synchronization of a frequency hopping transceiver to a network by embedding synchronization codes in the pseudo-random frequency hopping transmission sequence. A receiver is implemented with a frequency detector and a correlator to generate a correlator signal in response to the synchronization codes in the pseudo-random frequency detector and a correlator to generate a correlator signal in response the synchronization codes in the pseudo-random frequency hopping transmission sequence. Detection of a peak in the correlator signal is indicative of synchronization of the receiver with the network. The network entry synchronization scheme is such that, when two transceivers A and B are communicating, a third unnetworked transceiver C extracts the hidden network entry code pattern from the A-B transmission in order to enter the network. As a part of the communication between the two transceivers A and B, transceiver A transmits a known pattern as a hidden part of the communication which allows transceiver C to enter the A-B network. This hidden code pattern permits rapid synchronization and correction of large initial time errors, and permits correction of time drift from then on.
U.S. Pat. No. 5,081,641 issued Jan. 14, 1992 to Kotzin et al. entitled "Interconnecting And Processing System For Facilitating Frequency Hopping" discloses a method and apparatus for facilitating communication of information in a system without the use of a baseband hopping unit, by sharing a common TDM bus between a plurality of radio communication units, processing units, and information links, where the processing units extract traffic channel information, packetize and/or unpacketize the information, and return same back to the common bus for retrieval by the information links or radio communication units.
U.S. Pat. No. 5,079,768 issued Jan. 7, 1992 to Flammer entitled "Method For Frequency Sharing In Frequency Hopping Communications Network" discloses a frequency hopping communications system wherein frequency-hopping is implemented by the division of communication slots and the accumulation of slots into epochs, wherein each epoch equals the total number of available slots (number of channels times the number of time frames per channel). A transmitting node tracks the preestablished frequency-hopping pattern for its target receiver based on previously-acquired information. The transmission node identifies a receiver node. The transmission node then checks the frequency channel to determine if available (e.g., not in use and within an acceptable noise margin). If unavailable, the transmission node delays transmission to the identified node to a later slot. During the delay, the transmission node identifies another receiver node and a corresponding current frequency channel. The steps of identifying a receiver node and checking the corresponding current frequency channel are repeated until a node having an available frequency channel is identified. The transmission node then sends a packet to the selected receiver node at a frequency and for a duration defined according to the current slot. Such transmission node tracks the changing frequency of the selected receiver node to maintain frequency synchronization.
In U.S. Pat. No. 4,850,036 issued Jul. 18, 1989 to Smith entitled "Radio Communication System Using Synchronous Frequency Hopping Transmissions" a frequency-hopping radio communication system is disclosed comprising a control unit which transmits to and receives from each of a plurality of slave stations using a frequency-hopping mode of operation. During a start-up mode, the control unit communicates a starting message to each slave station using a predefined frequency. The message identifies to each slave station a frequency-hopping sequence to be used to select the frequencies from a group of frequencies for transmission to and reception from the control unit. This message also specifies to each slave station unique starting frequencies in the frequency-hopping sequence at which to begin transmitting and receiving. All slave station transmission are synchronized to the control unit transmissions, thereby preventing any two stations from concurrently using the same frequencies for either transmitting to or receiving from the control unit.
In U.S. Pat. No. 4,612,652 issued Sep. 16, 1986 to Kadin entitled "Frequency Hopping Data Communication System" an improved frequency hopping data communication system with a random transmission bandwidth to provide independent frequency hopping of the mark and space frequency is provided in the system which is particularly immune to repeater jamming. Only one frequency is transmitted at a time upon selection on a bit instant by a pseudo-noise code generator. The location of the mark and space frequency is randomly chosen, however, the location is known at the transmitter and the repeater by appropriate synchronization equipment.